Filter paper

The present invention relates to a wet-laid non-woven filter material comprising active carbon, submicron size glass fibers and binder fibers.

FR-A-2,216,377 discloses a filter material with the above-mentioned features for the use as a water filtration felt.

JP―A―81 129 038 discloses active carbon in form of a fiber for use in filters.

The need for highly efficient filter materials has long been recognized by both military and commercial personnel. The manufacture and use of certain dangerous chemicals such as pesticides, toxic materials etc., in the form of sprays or mists, gases etc., requires that personnel involved therewith be provided the safest protection available. Protection, e.g., in the form of gas masks, has, in the past, not been completely satisfactory in filtering both aerosols and particulates at a low pressure drop. Furthermore, most previously used filter materials do not provide toxic vapor adsorption as well as highly efficient mechanical filtration.

Accordingly, if a filter material could be developed which overcame the disadvantages of previous filters, a step forward in the art would be achieved.

According to the present invention, a non-woven filter material containing active carbon and submicron size glass fibers is produced via the wet-laying process, utilizing fibrillated fibers as the binder material, the filter providing excellent removal of aerosols, particulates and toxic vapors. The filters of the present invention have exhibited filtration index numbers over nine whereas previously used filters have exhibited values of slightly more than four.

In accordance with the present invention, there is provided a wet-laid non-woven filter material comprising active carbon, submicron size glass fibers and binder fibers, characterized in that the binder fibers are fibrillated.

The novel products of the present invention are prepared by wet-laying the active carbon, preferably in the form of fibers, submicron size glass fibers and fibrillated binder fibers from a water suspension thereof. The suspension should contain from about 2-20%, by weight, based on the total weight of the filter material, preferably from about 5-15%, by weight of the fibrillated binder fibers, from about 30-80%, by weight, same basis, preferably from about 40-70%, by weight, of the active carbon and from about 10-50%, by weight, same basis, preferably from about 15-40%, by weight, of the submicron size glass fibers, the total weight of the three components being 100%.

The active carbon, submicron size glass fibers, and fibrillated binder fiber are wet-laid using the conventional papermaking process well known in the art. Flocculating agents and surface active agents can be incorporated into the water suspension in order to facilitate the paper making procedure as is also known in the art. The binder fiber may comprise any materials known to be useful for this purpose, such as polyester fiber, polyolefin fiber, etc., however, the preferred fiber comprises acrylic fiber which may be produced from any known acrylic polymers such as polyacrylonitrile, copolymers of acrylonitrile and other copolymerizable monomers such as methyl methacrylate, ethyl acrylate, etc.; homopolymers and copolymers of other fiber forming monoethylenically unsaturated monomers such as vinyl acetate, vinyl chloride, styrene, vinyl pyridine, acrylic esters, acrylamide and the like.

Fibrillation of the binder fibers, which should range from about 1 mm to about 25 mm in length, is accomplished as is known in the art, i.e., such as by high speed shearing of the fibers.

The active carbon component of the novel filter of the present invention can comprise carbon particles or carbon fibers or mixtures thereof, carbon fibers being preferred. These carbon fibers are well-known in the art as are methods for their production. They can be used in lengths of from about 0.3 to about 15.0 mm, preferably from about 0.5 to about 10.0 mm, and can be prepared from such carbon fiber precursors as coal tar pitch, petroleum pitch, coal tar, petroleum derived thermal tar, ethylene tars, high- boiling coal tar distillates, ethylene tar distillates, gas oils or polynuclear aromatics. Also useful as precursors are polymers such as acrylonitrile homopolymers and copolymers, polyvinylalcohol, and natural and regenerated cellulose. Methods for preparing activated carbon fibers useful herein are disclosed in U.S. Patent Nos. 4,069,297 and 4,285,831.

The activated carbon powders or particles useful herein should have a particle size ranging from about 0.1 pm to about 100 um, preferably from about 1.0 ₁₁m to about 80 pm and may also be prepared from any of the carbon precursors described above.

The submicron size glass fibers useful herein are taught in the art and any known method for their production can be used to prepare them. The range in length is from about 1 pm to about 3500 pm, preferably from about 10 11m to about 2000 µm.

The wet-lay sheet making process (papermaking) used herein for the production of the novel filter material of the present invention, results in a filter having unique sorptive characteristics, a thickness of at least about 0.127 mm (0.005 inch), preferably at least 0.254 mm (0.01 inch), a high sorptive capacity to weight ratio and high porosity to fluid flow. The filter has superior filtering efficiency for aerosols and particulates and achieves this high efficiency at very low pressure drops. The filter has the capacity to remove toxic vapors from a gas stream by absorption and thus combines improved mechanical filtration through pores with absorption. The active carbon, when present as a fiber, tends to lay parallel to the plane of the sheet, and thus produces a long fluid flow path through the sheet which increases the time available to absorb impurities.

The novel filter material of the present invention may be exposed during or after its production to procedures to provide property enhancement. Thus, the surface of the novel filter material claimed herein may be rendered hydrophobic or oleophobic by treatment with such materials as methyltrichlorosilane, a polymer such as polytetrafluoroethylene, or PFOMA (1,1-dihydropentadecafluorooctylmethacrylate. Additionally, a reactive coating capable of decomposing toxic agents, e.g., a coating of sulfonated polymer to hydrolyze nerve gas, may be applied thereto so that the active carbon forms a second line of defense. Moreover, treatment of the filter with known mildewcides results in the production of mildew resistant materials.

The filter material of the present invention has a wide variety of uses. It is useful for protective purposes, i.e., for the filtration and separation of gases, aerosols and particulates. The uses include the manufacture of the filter material into components for air filtration systems, air filters for military vehicles and stationary facilities, gas mask cannisters and cartridges, gas purification filters, and the like.

The following examples are set forth for the purpose of illustration. All parts and percentages are by weight unless otherwise specified.

Example 1

A mixture of 10% fibrillated acrylic fibers, 80% activated carbon fiber and 10% Type 104 glass fiber in 18.1 of water are formed into a 30.5 x 30.5 cm (12" x 12") sheet using a standard hand paper making machine. The sheet is dried under pressure at 70°C to 120°C. The resultant filter material is then tested. The results are set forth in Table I, below.

Example 2

The procedure of Example 1 is again followed except that 10% fibrillated acrylic fiber, 65% activated carbon fiber and 25% Type 104 glass fiber are employed. The results of testing are set forth in Table I, below.

Example 3

The procedure of Example 1 is again followed except that 50% fibrillated acrylic fiber and 40% Type 104 glass fiber are employed. The testing results are set forth below in Table I.

Examples 4-6

The filter materials of Examples 1-3 are individually treated with 1,1-dihydropentadecafluoro- octylmethylacrylate and tested. The results are set forth in Table II, below.

In Tables I and II, below, thickness is measured on a TMI Model 551M micrometer gage. Tensile strength is evaluated on a Thwing-Albert Electro-hydraulic Tensile Tester Model No. 30LT. Porosity is calculated from measurements made using a Gurley-Hill S-P-S Tester and Dynamic Sorption is calculated based upon a threat concentration of 760 ppm CC1₄ in nitrogen, a downstream limit of 1.0 ppm CC1₄ and a flow density of 2.5 ml/min./cm². According to the equation:The Static Sorption is measured by determining the gain in weight of a sample stabilized in saturated CC1₄ vapor at room temperature divided by the starting sample weight.

Both Pressure Drop and Particle Smoke Penetration are measured on a Q127 DOP Filter Testing Penetrometer. This equipment maintains a flow velocity of 850 cm/min. (32 liters per minute through an area of 37.61 cm²). The Penetrometer generates dioctylphthalate (DOP) particles of 0.3 diameter average at test concentration of 100 mg/l.

The Filtration Index is a measure of the general effectiveness of a filter and takes into account the compromise between pressure drop and resistance to smoke penetration i.e.,where R = average resistance, millimeters of water

• P = average particle smoke penetration, %.